Piezoelectric transducer/detector arrays used in medical imaging comprise a number of transducers (typically 64 or more transducers) that are independently controlled to operate as a phased array. The phased array structure of such an ultrasound device allows the focusing of the ultrasound beam over a wide area of the body and retrieval of output signal from a large volume of the body. The resolution provided by an ultrasound array is a function of several factors, including the center frequency of the array, the bandwidth of the individual elements, the number of elements and their positions. A typical phased array design consists of multiple elements with their centers spaced by nor more than half the center frequency sound wavelength. Optimum performance requires good acoustic isolation between individual elements; such isolation is typically achieved by cutting (with a saw) material between the elements. It is, however, desirable to minimize the width of the isolation cuts in order to maximize the active area of the array. For example, the desirable spacing between elements in an array having a 7.5 MHz center frequency is about 100 .mu.m (0.1 mm) to avoid off-axis grating lobes which can decrease image contrast and resolution. Performance of the array is also enhanced when the transducer elements have a smooth finish.
Improvements in resolution are being sought from the use of higher frequencies (e.g., about 10 MHz to about 15 MHz) and an increased number of densely-packed transducers (e.g, in the range between about 128 to 256). Fabrication of high density arrays requires many steps of high precision and has proven to be difficult, time consuming, and expensive. For example, in one common fabrication method, a plate (or block) of piezoelectric ceramic material is diced part way through with a diamond saw or the like. Many precise saw cuts are necessary to define the individual elements in the array, which is a lengthy process. Other difficulties with this method of fabrication include the low physical strength of the array (the depth of the saw cuts affecting the structural integrity of ceramic block) and the size of the area separating respective elements being limited by the shape and thickness of the saw (most saws being of a size that the distance between array elements is greater than 25 .mu.m). The surface of the transducer elements formed by the cuts is typically rough, with variations in the surface plane of greater than 5%. Further, yield from such processes is low in light of the fact that one improper saw cut typically destroys the usefulness of the whole array.
Another method of array fabrication is disclosed in U.S. Pat. No. 4,939,826. In this method, a piezoelectric material is polished and cut to a desired size and then cut into small wafers. The wafers are placed in an assembly fixture to be stacked and bonded together. After application of backing material, the device is removed from the assembly fixture and some of the bonding material is removed from between the individual wafers. This process calls for precisely sizing the ceramic block at the beginning of the process, with the block being processed to produce one set of wafers for an array. Further, the nature of this process results in the handling of a very small workpieces for the majority of the processing in the construction of an ultrasound array; such work with small pieces is both tedious and susceptible to causing damage to array components, resulting in a reduced yield from the manufacturing process.
One object of the present invention is to provide a high yield method of fabricating a high density ultrasound array. Such an array typically includes a large number of transducer elements that have smooth finishes and has spacing between transducer elements of less than 25 microns.